Fine copper powder and process for producing the same

ABSTRACT

A fine flaky copper powder having an average major axis diameter of 4 to 10 μm and a flakiness of 2 to 20, has a bulk density of 2 to 4 g/cm 3  and a BET specific surface area of 0.4 to 1.5 m 2 /g; A process for producing the copper powder includes introducing a copper slurry into a medium type agitation mill and flattening the powder.

DESCRIPTION

[0001] 1. Technical Field

[0002] The present invention relates to fine copper powder and a processfor producing the same. More particularly, it relates to fine copperpowder which exhibits suitable characteristics when used in anelectrically conductive paste or an electrically conductive adhesive andan economical and convenient process for producing the same.

[0003] 2. Background Art

[0004] In the field of electronic components mounting techniques for OAequipment, portable communication equipment, etc., electricallyconductive pastes recently used in chips, contact bump materials, andthe like include those comprising a resin binder, glass frit, etc.having incorporated therein conductive metal particles or flakes mainlycomprising silver or silver-palladium. These conductive pastes are alsoused for through-holes, cross-overs, electrodes, etc. of printed wiringboards. The conductive pastes containing conductive metal powder orflakes mainly comprising silver or silver-palladium, while excellent inelectrical conductivity and resistance to oxidation, are disadvantageousin that such a metal powder as silver, palladium, etc. is expensive anddifficult to obtain stably and has a problem of migration resistance.Hence the need for copper powder that is cheap and excellent inelectrical conductivity has been increasing as a substitute forexpensive silver or palladium.

[0005] Currently adopted processes of producing copper powder includeatomizing, electrolysis, wet synthesis, and the like. Copper powderobtained by atomizing or electrolysis, which is chiefly used in powdermetallurgy, has an average particle size of about several tens ofmicrons. On the other hand, wet synthesis provides copper powder havingan average particle size regulated between about 0.2 to 4 μm with anarrow particle size distribution but involves high cost and has aneconomical problem.

[0006] With the electronic equipment and the like having been reduced insize and weight, the pitch of the conductive circuits therefor has beenmade finer. To cope with this trend, the copper powder to be used inconductive pastes for through-holes of printed wiring boards has beenrequired to be finer, specifically to have an average particle size of10 μm or smaller, preferably an average particle size between about 3and 5 μm. As mentioned above, although wet synthesis furnishes copperpowder having such a range of average particle size, it is economicallydisadvantageous and cannot be said to be an industrial method ofmanufacture. The copper powder obtained by atomizing generally has anaverage particle size of several tens of microns as described above. Ifsuch a copper powder is classified to obtain particles of 10 μm orsmaller, the yield is poor, and an increase in cost results.

[0007] To meet the above demand, copper powder having an averageparticle size of about 8 μm has been obtained by grinding electrolyticcopper powder having an average particle size of about 20 to 35 μm bymeans of an atomizer. However; still finer copper powder has beendemanded for use in conductive pastes. Use of a high water pressureatomizer could provide copper powder having an average particle size ofabout 5 μm, but the production yield is poor, which is economicallydisadvantageous.

[0008] Japanese Patent Application Laid-Open Nos. 199705/87 and182809/90 disclose crushing and pulverizing electrolytic copper powderby collision among copper particles give a fine copper powder of 10 μmor smaller in average particle size, that is, a method of crushing andpulverizing electrolytic copper by use of a jet mill of the system inwhich particles are made to collide with each other. This method,however, involves the problem that a conductive paste using theresulting fine copper powder is inferior in characteristics such aselectrical conductivity.

[0009] It is possible to obtain fine copper powder of 3 to 5 μm inaverage particle size by pulverizing electrolytic copper powder by meansof a jet mill of collision plate type. However, the fine copper powderobtained by this method comprises granular particles or a mixture ofgranular particles and twiggy particles. It is desired for the finecopper powder used in a conductive paste to comprise not only granularparticles, etc. but flaky or flat particles from the standpoint ofprevention of sagging and improvement of electrical conductivity.

[0010] Accordingly, an object of the present invention is to providefine copper powder having suitable characteristics for use in conductivepastes and conductive adhesives, particularly satisfactory electricalconductivity and anti-sagging effects, and an economical and convenientprocess for producing the same.

DISCLOSURE OF THE INVENTION

[0011] As a result of investigations, the present inventors have foundthat the above object is accomplished by pulverizing dendriticelectrolytic copper powder having specific properties and having beencoated with fat and oil by means of a jet mill of collision plate typeinto fine powder or by flattening fine granular copper powder by meansof a medium type agitation mill.

[0012] The present invention has been completed based on the abovefindings and provides a process for producing fine copper powdercharacterized by comprising mixing fat and oil into a dendriticelectrolytic copper powder having an average particle size of 20 to 35μm and a bulk density of 0.5 to 0.8 g/cm³ to coat the surface of theelectrolytic copper powder particles with the fat and oil andpulverizing the coated particles by means of a jet mill of collisionplate type.

[0013] The present invention also provides a fine flaky copper powdercharacterized by having an average major axis diameter of 4 to 10 μm anda flakiness of 2 to 20.

[0014] The present invention also provides a preferred process forproducing the above-mentioned fine flaky copper powder, which ischaracterized by comprising introducing a copper slurry of fine granularcopper powder having an average particle size of 3 to 5 μm dispersed inwater into a medium type agitation mill and flattening the fine copperpowder.

BRIEF DESCRIPTION OF THE DRAWINGS

[0015]FIG. 1 is a scanning electron micrograph of the dendriticelectrolytic copper powder of Example 1.

[0016]FIG. 2 is a scanning electron micrograph of the fine copper powderof Example 1.

[0017]FIG. 3 is a scanning electron micrograph of the fine flaky copperpowder of Example 3.

BEST MODE FOR CARRYING OUT THE INVENTION

[0018] The dendritic electrolytic copper powder which can be used in thepresent invention has an average particle size of 20 to 35 μm and a bulkdensity of 0.5 to 0.8 g/cm³, preferably 0.60 to 0.75 g/cm³. If theaverage particle size is out of the above range, satisfactory finecopper powder is not obtained. Where the bulk density is less than 0.5g/cm³, there is a fear that the powder may catch fire duringpulverizing. If it exceeds 0.8 g/cm³, fine copper powder having anaverage particle size of 3 to 5 μm cannot be obtained.

[0019] Such dendritic electrolytic copper powder is obtained by directelectrodeposition on a negative electrode in a liquid compositioncomprising 5 to 50 g/l of CuSO₄. 5H₂O and 50 to 150 g/l of H₂SO₄ at acurrent density of 5 to 10 A/dm² and at a liquid temperature of 20 to60° C. The electrolytically obtained dendritic copper powder issubjected to treatments, such as acid washing, water washing, and thelike.

[0020] Thereafter, fat and oil are added and mixed into the dendriticelectrolytic copper powder for rust proof treatment to coat the surfaceof the electrolytic copper powder uniformly with the fat and oil. Thefat and oil are preferably added in an amount of 0.1 to 5% by weightbased on the electrolytic copper powder. The fat and oil to be usedpreferably include saturated fatty acids and unsaturated fatty acids.The saturated fatty acids include lauric acid, palmitic acid, andstearic acid. The unsaturated fatty acids include oleic acid andlinoleic acid.

[0021] In the present invention, the oil/fat-coated dendriticelectrolytic copper powder is pulverized in a jet mill of collisionplate type. The jet mill of collision plate type is designed to make fedelectrolytic copper powder to collide against a target (collision plate)in the milling chamber by compressed air thereby to pulverize thepowder. The pulverized copper powder is classified by a classifier. Thedesired fine copper powder is discharged and collected by a cyclone or abug filter. Coarse copper powder is fed back to the milling chamber andpulverized again. Such jet mills of collision plate type include IDS JetMill manufactured by Nippon Pneumatic Kogyo K.K.

[0022] Japanese Patent Application Laid-Open Nos. 199705/87 and182809/90 supra disclose crushing and pulverizing dendritic electrolyticcopper powder by use of a jet mill of the system in which particles aremade to collide with each other, for example, PJM Jet Mill manufacturedby Nippon Pneumatic Kogyo K.K. However, the particles obtained by theuse of these methods are not so small as those obtained from the sameraw feed by the use of a jet mill of collision plate type. Besides, whenthe resulting fine copper powder is used in conductive pastes, etc.,such problems can occur that satisfactory electrical conductivity is notobtained.

[0023] Fine copper powder with a desired average particle size can beobtained by properly adjusting the speed of feeding electrolytic copperpowder and the cut-off size of the classifier of the pulverizer. Forexample, a fine copper powder can be obtained by lowering the feed rateand setting the classifier at a small cut-off size. A fine copper powdercan also be obtained by using electrolytic copper powder having a smallbulk density as a raw feed. In order to obtain fine copper powder havingan average particle size of 3 to 5 μm, it is desirable to slow down thefeed. To obtain fine powder having an average particle size exceeding 5μm, the feed rate is increased.

[0024] The fine copper powder thus obtained comprises granular particlesor comprises granular particles and twiggy particles and preferably hasan average particle size of 3 to 5 μm.

[0025] The fine copper powder obtained by the production process of thepresent invention can be used either alone or in combination with othercopper powders in conductive pastes or conductive adhesives for themanufacture of through-hole boards or electrodes of electroniccomponents. It is particularly suited for use in conductive pastes forthrough-hole boards having an increasing pitch of conductive circuits.

[0026] It is necessary for the fine flaky copper powder according to thepresent invention to have an average major axis diameter of 4 to 10 μmand a flakiness of 2 to 20. Having an average major axis diameter and aflakiness in the above respective ranges, the flaky copper powder of theinvention exhibits satisfactory conductivity and is effective inpreventing sagging when used in conductive pastes.

[0027] It is desirable for the fine flaky copper powder of the presentinvention to have a bulk density of 2 to 4 g/cm³ and a BET specificsurface area of 0.4 to 1.5 m²/g. Particles having these properties inthe above respective ranges have achieved suitably flattening andexhibits satisfactory conductivity and is effective in preventingsagging when used in conductive pastes.

[0028] The process for producing the above-described fine copper powderis explained below.

[0029] In the present invention fine granular copper powder having anaverage particle size of 3 to 5 μm is dispersed in water to prepare acopper slurry. The fine copper powder used here is not particularlylimited. While one obtained by atomizing or wet synthesis may be usable,it is preferred to use, for the economical consideration, one obtainedby pulverizing dendritic electrolytic copper powder obtained byelectrolysis by a jet mill of the system in which particles collide witha collision plate or with each other. One obtained by pulverizing bymeans of a jet mill of collision plate type is particularly preferred inview of the characteristics of a finally prepared conductive paste. Theterm “granular” as used above is intended to include not only particlesmainly comprising granular particles but those comprising granularparticles and twiggy particles. The average particle size of thestarting powder is limited to the above range because a fine flakycopper powder having a desired major axis diameter and a desiredflakiness cannot be obtained unless the average particle size is withinthe above range. It is desirable that the fine granular copper powder beuniformly coated with fat and oil, such as stearic acid, oleic acid,etc.

[0030] It is preferable to add lubricants, such as fatty acid salts,dispersants, and the like to the copper slurry having the fine granularcopper powder dispersed in water. The lubricant or dispersant added tothe copper slurry prevents the fine copper particles from sticking toeach other to have an increased size. Useful fatty acid salts as alubricant include sodium oleate, and useful dispersants include Emulgen910 (available from Kao Corp.).

[0031] The copper slurry having the dispersed fine granular copperpowder is introduced into a medium type agitation mill, and the finecopper powder particles are flattened. The medium type agitation milltypically includes a bead mill In using a bead mill having a capacity of1.4 liter, the copper slurry is preferably fed at a rate of 0.5 to 1.0l/min. Beads having a diameter of 0.3 to 1.0 mm are used. The beads canbe of ceramics, such as zirconia or alumina, glass, stainless steel,etc. In order to prevent copper particles from sticking to each other toincrease the particle size, it is desirable to use beads of smalldiameter and of small specific gravity. The operating time forflattening is usually from 30 minutes to 2 hours, while dependent on thedesired flakiness.

[0032] The above-described fine flaky copper powder is obtained in thismanner. The fine flaky copper powder according to the present inventioncan be used either alone or in combination with other copper powders inconductive pastes or conductive adhesives for the manufacture ofthrough-hole boards or electrodes of electronic components. It isparticularly suited for use in conductive pastes for through-hole boardshaving an increasing pitch of conductive circuits.

[0033] The present invention will now be illustrated concretely withreference to Examples, etc.

EXAMPLE 1

[0034] One part by weight of fat and oil (oleic acid) was added to 100parts by weight of a dendritic electrolytic copper powder having anaverage particle size of 28.2 μm and a bulk density of 0.63 g/cm³ andmixed for 2 hours. The mixture was pulverized into fine copper powder bymeans of a jet mill of collision plate type (IDS type jet mill, IDS-5,supplied by Nippon Pneumatic Kogyo K.K.) under conditions of 6 kg/cm³ inmilling pressure and 3.4 kg/hr in feed rate. The resulting fine copperpowder was generally granular and had an average particle size of 3.37μm. The recovery was 89%. Scanning electron micrographs of the dendriticelectrolytic copper powder used as a raw feed and the pulverizationproduct are shown in FIGS. 1 and 2, respectively.

[0035] Eighty-five parts by weight of the fine copper powder thusobtained, 16 parts by weight of a resol phenolic resin, and 6 parts byweight of a solvent (butyl cellosolve) were added up and milled intopaste on a three-roll mill. The paste was screen printed on anepoxy/glass laminate and heat-cured in an air oven at 150° C. for 30minutes. The conductive paste had a specific resistance of 1.2×10⁻⁴Ω·cm.

COMPARATIVE EXAMPLE 1

[0036] One part by weight of fat and oil (oleic acid) was added to 100parts by weight of a dendritic electrolytic copper powder having anaverage particle size of 14.4 μm and a bulk density of 1.34 g/cm³ andmixed for 2 hours. The mixture was pulverized into fine copper powder bymeans of a jet mill of collision plate type (IDS type jet mill, IDS-5,supplied by Nippon Pneumatic Kogyo K.K.) under conditions of 6 kg/cm³ inmilling pressure and 1.2 kg/hr in feed rate. The resulting copper powderwas generally granular and had an average particle size of 11.2 μm. Thatis, fine copper powder was not obtained. The recovery was as low as 78%.

COMPARATIVE EXAMPLE 2

[0037] A fine copper powder was obtained in the same manner as inExample 1, except for replacing the jet mill of collision plate typewith a jet mill of the system in which particles collide with each other(PJM type jet mill supplied by Nippon Pneumatic Kogyo K.K.). Theresulting fine copper powder had an average particle size of 5.8 μm, andthe recovery was 60%. The fine copper powder thus obtained was made intopaste, and the paste was screen-printed on an epoxy/glass laminate andthermally cured in an air oven in the same manner as in Example 1. Theconductive paste had a specific resistance of 3.0×10⁻⁴ Ω·cm.

EXAMPLE 2

[0038] One part by weight of fat and oil (oleic acid) was added to 100parts by weight of a dendritic electrolytic copper powder having anaverage particle size of 30.3 μm and a bulk density of 0.74 g/cm³ andmixed for 2 hours. The mixture was pulverized into fine copper powder bymeans of a jet mill of collision plate type (IDS type jet mill, IDS-5,supplied by Nippon Pneumatic Kogyo K.K.) under conditions of 6 kg/cm³ inmilling pressure and 6.7 kg/hr in feed rate. The resulting fine copperpowder was a mixture of granular particles and twiggy particles and hadan average particle size of 4.43 μm. The recovery was 97%.

[0039] Eighty-six parts by weight of the fine copper powder thusobtained, 14 parts by weight of a resol phenolic resin, and 6 parts byweight of a solvent (butyl cellosolve) were mixed up and made into pastein the same manner as in Example 1. The paste was screen-printed on anepoxy/glass laminate and thermally cured in an air. oven in the samemanner as in Example 1. The conductive paste had a specific resistanceof 1.0×10⁻⁴ Ω·cm.

EXAMPLE 3

[0040] Thirty parts by weight of a fine granular copper powder having anaverage particle size of 3.25 μm which was obtained by pulverizingelectrolytic copper powder in a jet mill of collision plate type, 70parts by weight of water, 0.2 part by weight of a lubricant (sodiumoleate), and 0.1 part by weight of a dispersant (Emulgen 910, availablefrom Kao Corp.) were mixed up and dispersed by agitation to prepare acopper slurry. The slurry was poured into a bead mill to flatten thefine granular copper powder particles. Dyno-mill (KDL-PILOT model,manufactured by Willy A. Bachofen AG, Switzerland; capacity: 1.4l) wasused as the bead mill. Zirconia beads having a diameter of 0.5 mm (beadpacking density: 80%) were used. The copper slurry was fed at a rate of0.7 l/min. The operating time was 1 kg/hr. The slurry was stirred andmade to circulate between the bead mill and the beads. As a result,there was obtained a fine flaky copper powder having an average particlesize of 5.97 pm, a flakiness of 10, a bulk density of 2.57 g/cm³, and aBET specific surface area of 0.89 m²/g. A scanning electron micrographof the resulting fine flaky copper powder is shown in FIG. 3.

[0041] Seventy-four parts by weight of the thus obtained fine flakycopper powder, 26 parts by weight of a resol phenolic resin, and 9 partsby weight of a solvent (butyl cellosolve) were added up and made intopaste on a three-roll mill. The paste was screen printed on anepoxy/glass laminate and thermally cured in an air oven at 150° C. for30 minutes. The conductive paste had a specific resistance of 0.9×10⁻⁴Ω·cm.

EXAMPLE 4

[0042] A fine flaky copper powder was obtained in the same manner as inExample 3, except for using a fine granular copper powder having anaverage particle size of 4.43 μm. The resulting fine flaky copper powderhad an average particle size of 7.01 μm, a flakiness of 10, a bulkdensity of 3.28 g/cm³, and a BET specific surface area of 0.76 m²/g.

EXAMPLE 5

[0043] A fine flaky copper powder was obtained in the same manner as inExample 3, except for using fine granular copper powder having anaverage particle size of 3.25 μm and, as beads, non-alkali glass beadshaving a diameter of 1.0 mm (bead packing density: 83%) and setting theoperating time at 2.0 kg/hr. The resulting fine flaky copper powder hadan average particle size of 5.36 μm, a flakiness of 10, a bulk densityof 2.38 g/cm³, and a BET specific surface area of 0.90 m²/g.

EXAMPLE 6

[0044] Thirty-nine parts by weight of the fine flaky copper powderobtained in Example 3, 39 parts by weight of a fine granular copperpowder having an average particle size of 4.78 μm, 22 parts by weight ofa resol phenolic resin, and 9 parts by weight of a solvent (butylcellosolve) were mixed up and made into paste in the same manner as inExample 3. The paste was screen-printed on an epoxy/glass laminate andthermally cured in an air oven in the same manner as in Example 3. Theconductive paste had a specific resistance of 0.8 ×10^(−4 Ω·cm.)

COMPARATIVE EXAMPLE 3

[0045] Seventy-eight parts by weight of a fine granular copper powderhaving an average particle size of 4.78 μm, 22 parts by weight of aresol phenolic resin, and 9 parts by weight of a solvent (butylcellosolve) were added up and made into paste in the same manner as inExample 3. The paste was screen-printed on an epoxy/glass laminate andthermally cured in an air oven in the same manner as in Example 3. Theconductive paste had a specific resistance of 1.0×10⁻⁴ Ω·cm.

[0046] Industrial Applicability

[0047] As described above, the fine copper powder of the presentinvention exhibits satisfactory electrical conductivity and is effectivein preventing sagging when used in conductive pastes, conductiveadhesives, and so forth. Further, the production process of the presentinvention provides such fine copper powder at low cost in a convenientmanner.

1. A fine flaky copper powder characterized by having an average majoraxis diameter of 4 to 10 μm and a flakiness of 2 to
 20. 2. The fineflaky copper powder as set forth in claim 1, which has a bulk density of2 to 4 g/cm³ and a BET specific surface area of 0.4 to 1.5 m²/g.
 3. Aprocess for producing a fine flaky copper powder, characterized byintroducing a copper slurry having fine granular copper particles havingan average particle size of 3 to 5 μm dispersed in water into a mediumtype agitation mill and flattening said fine copper powder.
 4. A processfor producing a fine flaky copper powder as set forth in claim 3,wherein said medium type agitation mill is a bead mill, and the beadsused in said bead mill are zirconia beads having a diameter of 0.3 to1.0 mm.